While the oyster harvest east of the Mississippi River has dropped precipitously in the years since the BP oil spill and an influx of fresh water from several Mississippi River diversions, white shrimp and blue crab fared much better there.

The oil spill in 2010, followed by the river diversions to fight the flow of the oil and the opening of the Bonnet Carré Spillway in 2011 due to flooding concerns, were likely large factors in successive poor oyster harvests, scientists and fishermen say. But not so for white shrimp and blue crabs, which typically survive better than oysters and brown shrimp in fresher water.

Shrimpers in the Lake Pontchartrain Basin, the area east of the river, hauled in 4.8 million pounds of white shrimp in 2012, a 44-percent increase over the annual average from 2002-2009, Louisiana Department of Wildlife and Fisheries data show. While white shrimp had dropped in 2011, the total 2011 and 2012 catch still places them about 5 percent above the pre-spill average.

And statewide, the white shrimp harvest in 2012 was 67.9 million pounds, right on par with pre-spill averages, according to preliminary numbers from the National Marine Fisheries Service.

Kim Chauvin, a shrimp processor and dock owner, described white shrimp as “really hardy, stronger than brown shrimp.” And scientists generally concurred, saying that while white shrimp still prefer saltier water to grow, they can survive in fresher water than their brown cousins.

And while a crab’s lifecycle often determines its salinity preferences, adult crabs are known to be able to survive in areas with no salinity at all.

Shrimp and crab often can swim away from environmental threats. Oysters, attached to their reefs, have no escape.

Still, with its influx of fresh water, Lake Pontchartrain Basin was the only major basin in which blue crab numbers increased in 2011 and 2012.

There, the blue crab catch rose by about 35 percent in 2011 and 15 percent in 2012 compared to the pre-spill average from 2002 to 2009, the data show. That’s a 25-percent rise on average in 2011 and 2012.

Crabbers meanwhile recently have said that 2013 has been a bad year in Lake Pontchartrain. The only official statewide catch numbers available are from January through March and do show a downturn compared to past years.

John Lopez, coastal sustainability program director of the Lake Pontchartrain Basin Foundation, has suggested various reasons, including oil, for the recent downturn. For example, he said hypoxic – low-oxygen – conditions could be a factor due both to more river water in the lake and the churning of the water during Hurricane Isaac in August 2012.

“We know that the crabs eat the clams and that the clams are depleted by the low oxygen,” Lopez said.

Julie Anderson, a Louisiana State University biologist who studies crab, also points to the longer winter in 2013 that might have kept crabs burrowed in the mud longer, conserving their energy and thereby not eating as much or growing as quickly.

Also because crabs were down in other areas across the state in 2011 and 2012, crabbers fished the Lake Pontchartrain Basin much more heavily than normal during those two years after the spill. In those two years combined, crabbers made 50 percent more trips in Pontchartrain Basin than during the pre-spill period. Anderson and some other scientists said that increased fishing effort might have led to overfishing and thus a possible dip in catch this year.

While overall Louisiana seafood catch has declined since the oil spill, any potential effects of the oil and dispersants themselves on these commercial species is difficult to pinpoint and generally scientists who have studied the matter say that other, more natural environmental factors likely are stronger drivers.

The federal and state government has made it clear, through extensive scientific testing, that Louisiana seafood is safe to eat, but many fishers and some scientists continue to fret about another question mark: the long-term reproductive health of the species.

Still, when examined in terms of the industry’s long-term ebbs and flows, the changes in seafood catch since the spill typically don’t differ substantially from other dips and valleys over the past decade or so.

In fact, in terms of white shrimp, it already was back even with its pre-spill average in 2012.

Crabbers statewide saw only small drops – a 6-percent decline in 2011 and an 8 -percent dip in 2012 compared to the pre-spill average. And, those 2012 numbers likely will increase because they still remain very preliminary.

Brown shrimp, however, are a different story.

Statewide, brown shrimp last year dropped to 28.7 million pounds, a 36- percent decline compared to the average between 2002 and 2009, according to preliminary 2012 data from the National Marine Fisheries Service. That’s by far the most substantial drop in a single year since the oil spill for any of these major commercial species.

Yet, when looked at over the past 10 years, 2012 was still a better year than 2008. That year Louisiana shrimpers only recorded 24.9 million pounds of brown shrimp – nearly 4 million pounds less than in 2012. So, scientists point out that the 36-percent dip in 2012 isn’t beyond general variability.

One factor 2008 had in common with 2011 and 2012: There was an influx of fresh river water in the estuaries, compliments of the Bonnet Carré Spillway, which was opened to alleviate flood concerns downriver.

View full sizeDays after the opening of the Bonnet Carre Spillway, the darker brackish water of Lake Pontchartrain, left, merges with the muddier fresh water from the Mississippi River, right, near the Causeway Monday, May 16, 2011. NOLA.com | Times-Picayune archive

Fishers and scientists often understand that fisheries numbers can and will fluctuate for a wide variety of environmental and economic reasons. Looking back at the past 52 years, for example, the average seasonal brown shrimp catch was about 30.8 million pounds, which suddenly makes that recent 28.7 million pounds of catch seem less awful.

And a necessary caveat is that National Marine Fisheries Service numbers often can rise, especially at this early stage.

For instance, the Fisheries Service last year originally had placed the state-wide 2011 brown shrimp numbers at 36.1 million pounds. Then earlier this year, the agency corrected them, indicating that Louisiana shrimpers actually had brought in about 39.3 million pounds to local docks.

And recent shrimp numbers available for 2013 also show that brown shrimp are improving.

Statewide catch for the month of May (the most recent month available) shows a stronger shrimp catch than any year since 2009. That month also was stronger than the May 2008 and 2007 catch, years before the oil spill.

Also, the National Marine Fisheries Service last week released preliminary numbers on Louisiana brown shrimp catch west of the Mississippi River, reporting that 31.3 million pounds had been caught during the recent season. That is about 2 percent above the average catch since 1960.

Processing freshly caught shrimp harvested from the waters south of Dulac, Louisiana at David Chauvin's Seafood on May 15, 2013. Chris Granger, NOLA.com | The Times-Picayune

When Mark Abraham, chairman of the state’s Shrimp Task Force, recently looked at the breakdown of shrimp catch numbers, he approached it by looking at how much shrimp in total was on the market in 2011 and 2012 – brown and white.

“It looks like it was just about a 10-percent drop in 2012,” said Abraham, who runs one of the largest shrimp processing companies in the state. “I think we'll catch up from that in 2013.”

“We feel that the shrimp industry as a whole will produce the same volume a year basically,” Abraham continued. “You might have some ups and downs, but we feel the average will be pretty constant.

“We have been having the same production in the Gulf for probably 50 years. I don’t think a 10-percent drop is that big a deal.”